The present invention relates to a steering system for non-tracked motor vehicles, having a steering handle, such as a manual steering wheel, operated by the driver, a steering adjusting drive for the steering adjustment of steerable vehicle wheels, a steering-angle desired-value generator operable by the steering handle, and a steering-angle actual-value generator operable by means of the steerable vehicle wheels. An automatic control system controls the steering adjusting drive as a function of a comparison of the desired and actual values of the steering angle and continuously monitors itself as well as a sensor system interacting with it with respect to a malfunctioning. A mechanical or hydraulic forced coupling is arranged between the steering handle and the steerable vehicle wheels, and is opened up or remains open (inoperative normal condition) when the automatic control system operates correctly and is automatically closed when the automatic control system is defective (operative special condition).
Steering systems of this type, which operate according to the "steer-by-wire" concept, are basically known and are developed for future motor vehicles. These systems offer the basic advantage that they are suitable at least with respect to the automatic control system as well as the associated sensor system without any constructive changes for many different vehicles. As a result of a corresponding programming, on one hand, virtually any transmission ratio can be implemented between the adjusting stroke of the steering handle and the steering angle change of the steered vehicle wheels. Furthermore, the automatic control system can be connected with additional sensors in order to be able to automatically take into account parameters to be defined, such as cross wind influences, or control deviations with respect thereto.
In order to be able to ensure the required amount of safety in the event of the occurrence of a defect in the automatic control system or in the event of a failure of signals which are to be analyzed by the automatic control system, an operating mode can be automatically switched on for an abnormal operation or an emergency operation. In this operating mode, a forced coupling is provided between the steering handle and the steered vehicle wheels so that the steering system operates in principle in the manner of a conventional steering system. The mechanical steering column, which is customary in conventional steering systems, can be replaced, however, by other mechanical systems or by hydraulic systems, particularly hydrostatic systems.